Tips for better image quality (Video Editing)
Blender uses a fast form of interpolation in the VSE preview, which can give a jagged result.

You don't really have to worry as this won't be in the final render, but it should also go away when you view the preview at 1:1 resolution so nothing needs to be interpolated. You can jump to 1:1 zoom by pressing Numpad 1 in the preview.

Not 1:1:  



1:1: